Chikungunya virus (CHIKV) is a mosquito-borne virus belonging to the Alphavirus genus of the Togaviridae family that was first isolated in 1953 in Tanzania, where the virus was endemic. Outbreaks occur repeatedly in west, central, and southern Africa and have caused several human epidemics in those areas since that time. The virus is passed to humans by two species of mosquito of the genus Aedes: A. albopictus and A. aegypti. There are several Chikungunya genotypes: Indian Ocean, East/Central/South African (ECSA), Asian, West African, and Brazilian.
Presently, CHIKV is a re-emerging human pathogen that has now established itself in Southeast Asia and has more recently spread to Europe. The Chikungunya virus (CHIKV) was introduced into Asia around 1958, and sites of endemic transmission within Southeastern Asia, including the Indian Ocean, were observed through 1996. The CHIKV epidemic moved throughout Asia, reaching Europe and Africa in the early 2000s, and was imported via travelers to North America and South America from 2005 to 2007. Sporadic outbreaks are still occurring in several countries, such as Italy, inflicting naive populations. Singapore, for instance, experienced two successive waves of Chikungunya virus outbreaks in January and August 2008. Of the two strain lineages of CHIKV, the African strain remains enzootic by cycling between mosquitoes and monkeys, but the Asian strain is transmitted directly between mosquitoes and humans. This cycle of transmission may have allowed the virus to become more pathogenic as the reservoir host was eliminated.
In humans, CHIKV causes a debilitating disease characterized by fever, headache, nausea, vomiting, fatigue, rash, muscle pain and joint pain. Following the acute phase of the illness, patients develop severe chronic symptoms lasting from several weeks to months, including fatigue, incapacitating joint pain and polyarthritis.
The re-emergence of CHIKV has caused millions of cases throughout countries around the Indian Ocean and in Southeast Asia. Specifically, India, Indonesia, Maldives, Myanmar and Thailand have reported over 1.9 million cases since 2005. Globally, human CHIKV epidemics from 2004-2011 have resulted in 1.4-6.5 million reported cases, including a number of deaths. Thus, CHIKV remains a public threat that constitutes a major public health problem with severe social and economic impact.
Despite significant morbidity and some cases of mortality associated with CHIKV infection and its growing prevalence and geographic distribution, there is currently no licensed CHIKV vaccine or antiviral approved for human use. Several potential CHIKV vaccine candidates have been tested in humans and animals with varying success.
Dengue virus (DENV) is a mosquito-borne (Aedes aegypti/Aedes albopictus) member of the family Flaviviridae (positive-sense, single-stranded RNA virus). Dengue virus is a positive-sense RNA virus of the Flavivirus genus of the Flaviviridae family, which also includes West Nile virus, Yellow Fever Virus, and Japanese Encephalitis virus. It is transmitted to humans through Stegomyia aegypti (formerly Aedes) mosquito vectors and is mainly found in the tropical and semitropical areas of the world, where it is endemic in Asia, the Pacific region, Africa, Latin America, and the Caribbean. The incidence of infections has increased 30-fold over the last 50 years (WHO, Dengue: Guidelines for diagnosis, treatment, prevention, and control (2009)) and Dengue virus is the second most common tropical infectious disease worldwide after malaria.
There is no specific treatment for DENV infection, and control of DENV by vaccination has proved elusive, in part, because the pathogenesis of DHF/DSS is not completely understood. While infection with one serotype confers lifelong homotypic immunity, it confers only short term (approximately three to six months) cross protection against heterotypic serotypes. Also, there is evidence that prior infection with one type can produce an antibody response that can intensify, or enhance, the course of disease during a subsequent infection with a different serotype. The possibility that vaccine components could elicit enhancing antibody responses, as opposed to protective responses, has been a major concern in designing and testing vaccines to protect against dengue infections.
In late 2015 and early 2016, the first dengue vaccine, Dengvaxia (CYD-TDV) by Sanofi Pasteur, was registered in several countries for use in individuals 9-45 years of age living in endemic areas. Issues with the vaccine include (1) weak protection against DENV1 and DENV2 (<60% efficacy); (2) relative risk of dengue hospitalization among children <9 years old (7.5× higher than placebo); (3) immunogenicity not sustained after 1-2 years (implying the need for a 4th dose booster); and (4) lowest efficacy against DENV2, which often causes more severe conditions. This latter point is a major weakness with the Dengvaxia vaccine, signaling the need of a new, more effective vaccine effective against DENV2. Other tetravalent live-attenuated vaccines are under development in phase II and phase III clinical trials, and other vaccine candidates (based on subunit, DNA and purified inactivated virus platforms) are at earlier stages of clinical development, although the ability of these vaccine candidates to provide broad serotype protection has not been demonstrated.
Zika virus (ZIKV) is a member of the Flaviviridae virus family and the flavivirus genus. In humans, it causes a disease known as Zika fever. It is related to dengue, yellow fever, West Nile and Japanese encephalitis, viruses that are also members of the virus family Flaviviridae. ZIKV is spread to people through mosquito bites. The most common symptoms of ZIKV disease (Zika) are fever, rash, joint pain, and red eye. The illness is usually mild with symptoms lasting from several days to a week. There is no vaccine to prevent, or medicine to treat, Zika virus.
Deoxyribonucleic acid (DNA) vaccination is one technique used to stimulate humoral and cellular immune responses to foreign antigens, such as ZIKV antigens. The direct injection of genetically engineered DNA (e.g., naked plasmid DNA) into a living host results in a small number of its cells directly producing an antigen, resulting in a protective immunological response. With this technique, however, comes potential problems, including the possibility of insertional mutagenesis, which could lead to the activation of oncogenes or the inhibition of tumor suppressor genes.